Method and means for hobbing spur gears



*March 13, 192s. 1,662,408 I N. TRBOJEVICH METHOD AND MEANS FOR HOBBING SPUR GEARS n Filed March 24. l1924 a sneetssneet March 13, 1928. 1,662,408

. N. TRBOJEVICH METHOD AND MEANS FOR HOBBING SPUR GEARS Filed Maron 24. 1924 3 sneetssneet2 March 13, 1928. y 1,662,408-

N. TRBoJEvlcll METHOD AND MEANS FOR HOBBINGSPUR GEABS Filed Maron 24,1924 5 sheets-sheet 5 Ivm,

o T I l 27 Q ,3;

Patented Mar. 13, 1928.

UNITED STATES PATENT OFFICE.

NIKOLA TRBOJEVICH, OF DETROIT, MICHIGAN, ASSIGNOR TO GLEASON WORKS, Ol"

'y ROCHESTER, NEW YORK, CORPORATION OF YORK.

/ z METHOD AND MEANS 'FOR HOEBING SPUR GEARS.

Application led March 24, 1924. Serial No. 761,527.

The invention relates to a novel modification of or improvement in the well known spur gear bobbing process by Whlch the cutting time for each gear may be reduced,

while at the same time gear teeth of superior finish and greater smoothness may be produced than was heretofore possible by the common bobbing method. Another advantage of the new process is that it is applicable to the present spur gear bobbing machines without any material changes being needed in said machines. Still another advantage is that the opposite sides of the gear teeth may be finished simultaneously byrtwo different hobs, which fact permits of adjusting the two hobs in the machine to generate with great accuracy any desired thickness of teeth within certain limits.I

In carrying outm new method I preferably employ a singe arbor on which are mounted a pair of frusto-conical hobs having their smaller ends adjacent, each of saidv hobs having teeth arranged ina helix which in plane development forms an Archimedean spiral. The hobs are also designed to have a constant lead and to form in axial section a rack of constant pitch,'being similar in this respect to the hob for generating spiral bevel gearing disclosed in my prior Patent Number 1,465,151, issued August 14, 1923. Such hobs, however, have never heretofore 4been utilized in the generation of spur gears, but I have discovered that when properly designed for this purpose they have a favorable cutting vaction which makes them particularly adaptable for this use.

In the drawings Figure 1 shows a common ear hob;

Figure 2 shows an elevatlon of tapered hob used in this process;

Figures 3 and 4 illustrate the arrangement of tool marks upon a hobbed surface;

Figure 5 is a fragmentary side view of a tapered hob, showing hooked gashes or flutes; y

Figure 6 shows two tapered hobs in engagement with a spur gear;

Figure 7 is a transverse section on -the line 7-7 of Figure 6;

Figures 8 and 9 are diagrams showing the generation of a helical gear by' the new process. v

The theory of the new process will be understood without vdiiiculty by all those who are familiar with the common or s ur bobbing process. In that process a spur ob A (Figure 1) having an axial rack element g, a, plurality of cutting teeth aligned along a circular helix, and a ressure angle of cutters a, is used. There ore, when said hob is rotated with a uniform angular velocity as indicated by the arrow, the rack generator g will` move with a constant linear velocity in a dlrection parallel to the axis of the hob, the ratio of the two velocities for single threaded hobs being such that for each revolution of the hob about its axis the rack generator g advances a distance equal to the pitch, or the distance between two consecutive teeth, ofthe generator g. Re arding the normal curvature of the cutting elicoid it should be noted that the opposite sides of hob teeth possess the same curvature, the radius of which (after neglecting the infiuence of the usually very small angle of helix) is equal to:

r sin a Where R is the radius of curvature and r the pitch radius of the hob. Assuming a value of a=20 degrees itis seen that the radius of curvature of such hobs is about 2.9 timesl greater than the radius of the hob.

I have discovered that a conical or tapered hob B (Figure 2) while possessing the rack element g and the constant ratio between the rotation of the hob and the linear advance of said rack element, similar to the spur hob, has different radii of curvature on the op osite sides of the thread. Thus, said r 'us for sides of teeth facing theapex of 4the hob 1s:

and for the sides farthest from the apex,

r R2: sin (oa-H3) formerly used without destroying the smooth finish of the bearing tooth surfaces. In Figure 3 the finish of a hobbed tooth surface is diagrammatieally shown disclosing a number of so called flats, the latter being arranged in a series of parallelograms of a width fo and length f, said dimension f being equal to the feedper revolution of gear blank. As is well known, the distance f., primarily depends upon the number of flutes or gashes in the hob, while the depth L (Figure 4) of the flats is independent of the latter distance fu and may be approximately calculated from the formula where f is the feed per revolution of blank and R the normal radius of curvature of the hob tooth, both in inches. The formula 1 shows that, other things` being equal, the feed of a hob may be increased in a square root proportion with the radius of curvature of its thread. Or, assuming the above indicated dimensions of the tapered hob, such a hob may (theoretically) take a feed 11A times greater than a similar sized spur hob.

In addition to this superiority of the tapered hob over the spur hob there are also a number of other advantages in its use. On account of the extreme tlatness of the generating helicoid (which tlatness may be increased or regulated at will by suitably selectingr the cone angle of the hob) the new hobs may be made to have hooked teeth (Figure without measurably affecting the accuracy of the tooth form, and in such cases the hook angle e which may vary from 5 to 12 degrees, improves the cutting etliciency of the hob. As the hob teeth are expected to finish the teeth in the blank only with their sides lying nearest to the hob apex, such hobs may be ground in the thread after hardening with less ditiiculty than the ordinary hobs, as only one side of each tooth has to be closely watched during the grinding operation. Also such hobs are not very sensitive in the case of run-out of the hob arbor, because the generating faces are very nearly perpendicular to the hob axis. Most important of all, however,the new hobs do not contact with the gear blank in the line of shortest distance connecting the hob and the blank axes, as in the case of common hobs, but at an offset point, which fact permits of mounting two hobs upon the same arbor, providing the diameter of the gear blank is neither too small, nor too large.

In Figure 6 such an arrangement is shown. Two tapered hobs 21 and 22 of the same required pitch and cone angle are first made and mounted upon the hob arbor 23 so that they face each other with their small ends and are separated by a spacing collar 24 of a. suitable width. Both hobs are of the same hand (preferably right hand) and are tlutedand relieved so that they eilt in the same direction. Thus when the hob arbor 213 is rotated with a uniform velocity, the two rack elements g, and ,f/g will move in straight lilies also with a uniform velocity, as indicated by tht` arrows. The gear blank 25 is placed so with respect to the two hobs 21 and 22, that its pitch cylinder 2G is tangent to the V formed by the two rack elements y, and y: at the points C, and C.: which two points lie substantially in the middle of the corresponding hobs 21 and 22.

It is now necessary to adjust the hobs on the arbor so that their helixes correctly mesh with the teeth of the blank 25, in order that the faces of the gear teeth lying to the right hand side (as indicated in the drawing) may mesh with and be generated by the tlat helieoid of the hob 21, and the faces lyingr toward the left hand .side by the fiat helicoid of the hob 22. In order to lo this, thel hob 2l is permanently keyed to the arbor by meansl of the key 2T fitted into the arbor keyway 28, and is pressed against the accurately ground boss' 29 of the arbor 225. The other hob is not keyed directly to the arbor, but is driven by means of a dog Pit) keyed to the arbor 23 by means of a key ill and engaging the flat pin 32, screwed into the hob 22. It'is readily seen that if the hob 2l is held relatively immovable the arbor and the hob 22 is rotated on the arbor by removing theI dog` 30, eventually a position of tht` hob will be found (on account of the helical thread) in which both hobs correctly mesh with the blank 25 and produce teeth of the required exact thickness and depth. ln that position the second hob 22 is then fixed and held by means of the two set screws ll and 34 housed in the dog 30. The six splines 35 in the bore of the dog il() serve for a rough or approximate adjustment. while the set screws 32B and 3f serve for tine adjustment.

After the two hobs 2l and 22 and the gear blank 25 have thus been correctly adjusted, the whole hob assembly is tightly clamped together b v means of the hexagon nut 36 and washer 3T.

It should be noted that the axis of thc pair of hobs is not exactly parallel to the plane of the blank 25, but is tipped with respect to the latter to a small angle corresponding to the medium thread angle of the hobs employed. It is also readily `seen that this method is applicable not only to the generation of spur gears having straight' teeth but also to those having helical teeth, in which case the hob arbor is tipped with respect to the plane of rotation of the blank corresponding to the desired helical angle to be generated. Thus, in that respect the new method is similar to the common hobbing process.

In Figure 8 and Figure 9, the above delll) scribed conditions are schematically presented, Fi ure 8 lshowing the elevation, and Figure 9 t e plan view ofthe relative position in a hobbing machine of a pair of conical hobs 21 and 22 and the helical gear blank 38. The procedure'of setting up the hobbing machine and the conicalhobs thereon usually starts from the fixed distances and angles Such as the pitch radius of the blank ro, the cone angle y0 of the tapered hobs, andthe helical angle 80, of the gear to be generated. From this data the distance X between the two hobs and the `Iangle of inclination 3 of the hob arbor may be determined by calculation, graphically or experimentally. After the hobbing machine has been correctly set up, `the hobs and the blank are rotated in the required timed relation, and the hob arbor is translated in a direction parallel to the axis of the blank.

The steps constituting the new method,

may now be summarized. First a pair of tapered hobs of constant pitch are made having the required pitch and pressure angle. There is no limitation as to the diameter of the hobs, in fact, the larger the hobs are the more nearly will the generated tooth curve approximate the theoretically correct involute. Similarly, the cone angle may be selected within certain limits, the preferred selection being about one degree less than the pressure angle of the rack generator. VSuch hobs are capable of producing various numbers of teeth and various helix angles in gear blanks, although the width of the spacing collar 24 separating the two hobs (Figure 6) must be roperly selected in each case.` After assemb ing the hobs on the hob arbor, the cutter head of the hobber is tipped to the required angle of helix and the hob arbor is longitudinally centered until both rack elements touch the blank. The cutter head is then raised and the blank fed in to ther desired depth of cut, the hobs are angularly adjusted on the arbor to track in the same helical path as already described, and the blank is ready to be generated. As shown in Figure 6 the cutting teeth in both hobs are considerably thinner on the pitch line than the width of space of gear teeth to be generate This 'mayA be considered as an additiongluadvantage of the process, as the same hobs/ ay be used both for roughing and also for finishing operations.v

The manufacture of the conical hobs used in this method including the operation of turning, boring, thread milling, gashing, relieving, hardening, grinding and sharpening, has already been described inmy above mentioned patent, to which the reader is referred for those particulars.

It is also `obvious that the method may be subjected to countless trivial modifications and variants without departing from the spirit of this invention. Thus, instead of having two hobs of the same cone angles, said two cone angles might be different, and yet the hobs would work. Or, instead of generating the gear teeth with the flat helicoids of the hobs, the opposite, pronouncedly convex helicoids might Abe used for that purpose, or again, both helicoids might be used simultaneously. Again, one of the hobs may be. omitted, and the gear teeth iinished with only one tapered hob. The hobs themselves might be made with inserted teeth, multiple threads, spiral flutes, etc.

What I claim as 1n invention is 1. A method of hob ing spur gears which consists in mounting two tapered hobs upon a common arbor in such a manner as to simultaneously Contact with a gear blank, each of said hobs having the cutting portion helically arranged on the tapered portion thereof in more than one convolution, and in rotating said hobs and blank in timed relation to simultaneously generate. opposite sides of the teeth with the respective hobs.

2. A method of hobbing spur gears which consists in rotating two tapered hobs in each of said hobs having a plurality of cutting teeth helically arranged on the tapered porti-on thereof in more than one convolution in arranging said hobs in such a manner that one side of the gear teeth is generated by one `hob and the other side of said teeth is generated by the other hob during the saine cutting operation and in imparting a relative movement of feed.

3. A method of generating gears consisting in arranging two tapered hobs upon a common arbor with the small ends adjacent', each of said hobs having a plurality of cutting teeth helically arranged on the tapered portion thereof in more than one convolution, mounting said arbor in such a manner that the hobs are tangent to a gear blank at two offset points, and rotating the hobs in timed relation to the gear blank.

4. A method of generating gears which consists in mounting two tapered hobs upon a common varbor in such a manner as to simultaneously engage a gear blank in tangential relation thereto, each of said hobs having ytimed relation to a cylindrical gear blank,

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a plurality of helically arranged cutting teeth on the tapering portion thereof in more than one convolution, and in rotating said arbor and gear blank in a timed relation to continuously cut the teeth in said blank by the simultaneous action of said hobs.

5. A method of hobbin gears which consists in selecting a pair o hobs having teeth helically alignedon the face of a frusto-conical body in more than one convolution, said teethhaving opposite sides of different radii of curvature, in rotatin said hobs in timed relation to a gear blan and in arranging the hobs in such relation to the gear blank that the hob teeth having greater radii ofy curvature generate the opposite sides ot the gear teeth 6. A method of gear bobbing consisting in selecting two conical hobs each having a plurality ot' cutting sections in the form of a series of rack cutters of constant pitch helically aligned upon the face of a frustolconical body, said hobs being exactly alike with regard to their respective cone angles, helical and pressure angles, hands and leads of helix and the direction of cut, and substantially alike with regard to their diameters and width of face, in further so selecting the cone angle of the hobs that it is slightly less than the pressure angle of the rack cutters, in aligning said hobs with the blank until said blank contacts with both hobs at two s vmmetrically disposed and ott'set points, and the hob teeth track in the intcrdental spaces of gear teeth, in rotating the hob and work arbors in a timed relation, and in imparting a relative movement of tccd to the hob arbor in a direction parallel to the axis ot' the blank.

7. A method of bobbing spur gears consisting in selecting a hob having a pluralityv of cutting teeth aligned on the face of a truste-conical body in a helical path in such a manner that they form in plane development an Archimedean spiral, and a rack of constant pitch in the aXial plane sections of said body, in bringing said hob into tangency with the blank at an otl'set point of the pitch cylinder of said blank, in rotating the two elements in a timed relation, and in imparting to the hob a relative movement of translation in the common tangent plane, to generate all gear teeth in their entire lengths in a continuous operation.

8. A method of hobbing spur gears consisting in ,select-ing a conical hob-having a plurality of cutting sections in the form ot of a plurality of rack elements of constant pitch helically aligned on the face of a frusto-conical body, the cone angle of said body being such as not to exceed the pressure angle of the rack elements, in bringing said hob in tangency with the blank at an offset point, in rotating the two elements in a timed relation and in imparting to the hob a relative movement of feed to generate all gear teeth in their entire length and depth in a continuous operation.

9. A method of bobbing spur gears con sisting .in mounting a truste-conical hob having cutting teeth helically arranged on the tapered body portion thereof in a plurality of convolut1ons of constant pitch upon the hob arbor of a common bobbing machine, and a cylindrical gear blank upon the work arbor of said machine, in bringing the hob in a tangent relation with the blank at an otset point, in tiltincr the hob arbor with respect to the plane ot? blank to an angle corresponding to the respective helical angles of the hob thread and gear teeth to be generated, in rotating the two elements 1n a timed relation and in feeding the hob in a direction parallel to the work axis to complete the teeth.

l0. A method of hobbing gears consisting in selecting two truste-conical hobs each having cutting teeth helically arranged on the tapered body portions thereof in a plurality of convolutions of constant pitch, in placing said hobs in such a relation upon a hob arbor of a bobbing machine that their small ends tace the central plane of the machine and cach other, in adjusting the longitudinal distance between said hobs to correspond to the diameter of the blank to be generated. in tilting the hob arbor with rcspect to the plane. of rotation of the blank to an angle corresponding to the respective helical angles of the hob thread and gear teeth to be generated, in bringing the blank into tangency with hobs so it will contact with the hobs at two offset points situated on the opposite sides of the central plane of the machine, in adjusting the hobs circumferentially in order that their respective threads track in the same helical path syllchronouslv with the rotation of the blank, in adjusting the distance between the hob and work arbors corresponding to the required depth ot cut, in rotating the two arbors in a timed relation and in imparting to the hobs a relative movement of feed in a direction parallel to the axis of blank.

ll. A method ot bobbing gears consisting in selecting two frusto-conical hobs each having cutting teeth helically arranged on the taper body portion thereof in a plurality of convolutions and having the same conc angles, hand of helix and direction of cut, and capable ot correctly meshing with the same rack generator o constant pitch, in placing said hobs upon a hoh arbor of a hobbing machine in such a relation that their small ends face each other and their conical faces contact with a gear blank at two offset points symmetrically disposed with respect to the central plane of the machine, in angularly adjusting the hob arbor about an axis in the central plane, and the hobs upon said arbor so that both hobs and the gear teeth to be generated track in continuous helical paths and mesh with the same system of imaginary rack planes, in rotating the hobs and the blank in a timed relation nnd in imparting a relative movement of feed to the hob arbor along a line parallel to the axis of blank.

12. A gear generating tool comprising an arbor, a pair of frusto-conical hobs each having cutting teeth helically arranged on the tapered body portions thereof in a lurality of convolutions mounted on said ar r, said hobs being adjustable longitudinally and rotatable with respect to each other and being provided with helically aligned gear teeth having different radii of curvature on opposite sides thereof.

13. A gear generating tool comprising an arbor, a pair of tapered hobs each having cutting teeth helicall arranged on the tapered body portions t lereo in a plurality of convolutions mounted thereon, means for directly driving one of said hobs from said arbor, and rotatable adjustable means for driving the other hob.

y 14. A gear generating tool comprising an arbor, a pair of tapered hobs mounted thereon with their smaller ends adjacent, means for locking one of said hobs to said arbor, the other hob being rotatable on said arbor, driving means between said arbor and the rotatable hob, said driving means including means for rotatively adjusting the angular' position of said rotatable hob` with respect to said arbor.

15. A gear generating tool comprising a pairl of cutters,'each having a plurality of cutting sections in the form of a series of rack elements of constant pitch helically arranged upon the tace of a truste-conical body, and means for mounting said cutters for simultaneous rotation about a common 'axis inv such a manner as to be capable of simultaneously generating the opposite sides of the teeth ot a spur gear.

16. gear generating tool comprising an arbor and a pair of :truste-conical hobs mounted on said arbor, each hob having cutting teeth helically aligned on the conical tace thereof in a plurality of convolutious in a path forming in plane development an Archilnedean spiral, said hobs being so arranged on said arbor as to be capable ot generating the opposite sides of the teeth otl a spur gear.

17. A tool for generating spur gears comprising an arbor. a pair of truste-conical hobs on said arbor having their smaller ends facing each other, said hobs having generati'ng elements helically arranged on the conical surtaces thereof and forming in axial section racks ot constant pitch, the smaller ends of said hobs being spaced axially of said arbor and said generating elements being. arranged in predetermined relationship such as to be capable of simultaneously generating the opposite sides of the teeth of aspur gear. K

In testimony whereof I afiix my signature.

NIKOLA TRBOJEVICH. 

